Video has extra "ghost" window

Running Zoom on an HP Laptop, using Linux Mint, latest version (dowloaded yesterday). There's a long-standing problem of video suffering from jitter and lag. Other machines are fine.

This machine has two video adapters - I'm only using one for Zoom. This is how the  OS sees the hardware:

  Device-1: AMD Stoney [Radeon R2/R3/R4/R5 Graphics] vendor: Hewlett-Packard
    driver: amdgpu v: kernel ports: active: eDP-1 empty: DP-1,HDMI-A-1
    bus-ID: 00:01.0 chip-ID: 1002:98e4 class-ID: 0300
  Device-2: AMD Topaz XT [Radeon R7 M260/M265 / M340/M360 M440/M445 530/535
    620/625 Mobile]
    driver: amdgpu v: kernel pcie: speed: 2.5 GT/s lanes: 4 bus-ID: 01:00.0
    chip-ID: 1002:6900 class-ID: 0380

One strange features is that when I close the Zoom window, another zoom window behind is revealed, and closes.  This "ghost" window is faint,  translucent and offset from the main window. More details are hard to see as the window disappears in less than a second.

I have never seen this "ghost" on any other machine - Linux or Windows. In case it was useful I have checked that the setting for dual monitors is off.

Any ideas on where to start with this problem?

Gordon